This invention relates to sensors for detecting the motion of eddy wheels in electric power meters, and in particular to a dual-emitter single detector sensor with a digital filter.
One of the most widespread techniques for measuring the electric power supplied to homes and businesses is the use of an electric meter employing an eddy wheel. Meters employing this technology are commonly termed watt-hour meters.
A type of watt-hour meter in common use for the measurement of alternating current is an induction meter. An induction includes two electromagnets with the coil of one being energized by the current consumed, and with the coil of the other connected to the consumer voltage. Because the current and voltage in the consumer circuit are in phase with each other, the current in the voltage coil and therefore the magnetic field of that coil will lag 90.degree. in relation to that of the current coil. The result of the interaction of the two coils produces a moving magnetic field which induces eddy currents in a light rotor disk. These currents cause the disk to rotate in the direction of motion of the moving field. The speed of rotation of the disk is thus proportional to the strengths of the two magnetic fields, and therefore the number of rotations of the eddy wheel is proportional to the power consumed. A small braking magnet is typically employed in such meters to constantly produce eddy currents in the rotor. These currents damp the rotation and assure that the rotor stops when no power is being consumed.
Meters employing this technology are in widespread use in the U.S. and abroad, with literally millions sold. Such meters are commonly read by a human observer noting the position of a series of counting wheels on the face of the meter and then supplying that information to the utility system which provides the power. The difference in count between two periods indicates the power consumed, and therefore enables the utility company to generate an appropriate bill for the installation at which the meter is situated.
Because of the inaccessibility of some power meters, and because of a desire to minimize the labor intensive practice of manual reading of the meters, various automatic meter reading systems have been developed. Such systems remotely detect the meter count and using that information enable utilities to generate billing information without personal observation of the meter dials. Furthermore, an increasingly common practice is demand metering in which the charge for electric power consumed varies according to time of day, season, or other factors. For such demand metering or remote meter reading, it is essential that the meter values be determinable at arbitrary, possibly frequent intervals. Of course, such demand essentially eliminates the ability of a human observer to detect the position of the meter dials, necessitating remote reading of the meter.
Another increasingly common practice is load shedding. In load shedding, consumers pay varying rates for electricity being supplied with an understanding that the utility may restrict the amount of power available or even disconnect power during peak demand periods. These practices also necessitate the remote reading of meters.
As a result, it is increasingly desirable to be able to detect remotely the amount of power consumed at relatively frequent and possibly short intervals. This need has led to the development of meters which supply information remotely using a variety of techniques. The use of such meters, however, presents an enormous cost to a utility which, to employ such meters, must replace existing meters. Accordingly, a need exists for the development of "retrofit" apparatus for mounting on existing watt-hour meters to detect at the meter the power consumption at intervals as frequently as desired.